Cassinis, R.

Assuming as a starting model the pattern of the Moho boundary as interpreted in a recent study on the basis of the available DSS profiles, a preliminary 3D gravity model of the crustal structures in the Central-Eastern Alpine sector is proposed. The aim of the present work is to confirm the seismic results concerning the Moho and to better shape the main discontinuities in the intermediate and upper crust, where the seismic data are too scattered to allow a reliable interpretation. The gravity field is calculated along twelve cross-sections oriented S-N and crossing the Alpine range from the Padan-Venetian plain to the Bavarian molasse and to the Austrian calcareous Alps. The westernmost section coincides with the European Geotraverse while the easternmost one is positioned at the longitude of about 14ºeast. The assumed density model is very simple (only 6 layers); for each unit the density is maintained constant. The model describes a European mantle dipping southwards underneath an overlapping, uplifted Adriatic mantle. As far as the lower crust is concerned, its top is found at depths between 18 and 28 km, the deepest values being reached in the south-eastern sector; the density appears higher in the Adriatic domain than in the European one and the Adriatic lower crust seems to be deeply indented northwards. The low density surface layers appear very thin in a large area of the northwestern sector, while in the south and southeast their thickness reaches about 10 km. This study must be considered as a complement to the seismic interpretation both as a validation of the model of the deep crust and Moho boundary and as an additional source of information on the upper crust.

Both historical and instrumental updated catalogues of the earthquakes in the Italian region, are used to compare the location of the epicentres and the focal depth of seismic events with the structure of the Earth crust as determined by the Deep Seismic Soundings (DSS) and according to a published interpretative synthesis. In the past, several attempts were made to do this on the same subject; however, the investigations concerned only some areas of the region, when both the seismic activity and the interpretation of DSS profiles were often based upon inhomogeneous or preliminary data. In this work, we profit from the most recent and complete database of Italian seismicity, to extend the study to the whole peninsula. The comparison confirms that the seismicity occurs more frequently where structural changes are observed by the DSS profiles. The main activity seems to originate in the upper, brittle crust; nevertheless, even the shallow events are particularly concentrated where tectonic features were discovered in the lower crust or at the Moho boundary. The hypocenters tend to deepen where a process of active subduction is occurring. It is believed that the results described herewith could be used as additional elements for the study of seismogenesis.

Questo studio rappresenta la conclusione di una serie di analisi precedentemente proposte e volte alla migliore comprensione della struttura profonda della penisola italiana. Tale serie, iniziata con il confronto tra la sismicità crostale e le interpretazioni dei profili DSS (Cassinis e Solarino, 2004; Cassinis e Solarino, 2006), si era successivamente estesa alla struttura litosferica (Solarino e Cassinis, 2005; Solarino e Cassinis, 2007). In ambedue le occasioni lo studio era stato condotto avendo cura di utilizzare le più recenti elaborazioni dei profili e selezionando gli ipocentri con criteri molto restrittivi. Nello spirito dei precedenti lavori, anche in questo caso si è eseguita una analisi congiunta di dati ottenuti con diverse tecniche sovrapponendo alle interpretazioni già esistenti le informazioni ricavate da un recente studio tomografico (Scafidi et al., 2008) effettuato con la tecnica della inversione di terremoti locali (Thurber, 1983).

In a recent paper we compared the earthquake hypocenters, plotted according to updated catalogs, with the structure of the earth’s Crust interpreted after the results of seismic exploration (mainly the Deep Seismic Soundings – DSS). The comparison was made along several cross sections in the Alpine range, the Italian Peninsula and the surrounding seas. The main conclusions of this analysis were that 1) the majority of the events is positioned in the upper, rigid crust and 2) the earthquakes tend to concentrate above the discontinuities unveiled by the seismic exploration in the deep crust and at the Moho boundary. With the goal to shed some light on the continuation of these structures with depth, in this paper a similar analysis is conducted even in volumes where DSS information are not available. It is apparent that the upper mantle seismicity is very unevenly distributed; therefore we only focus on the areas where a sub-crustal seismicity is recorded, adding to the seismic models of the crust some information, if available, on the physical characters of the upper Lithosphere. Four areas are examined: the well known Calabrian (Aeolian) Arc where the Ionian plate is subducted beneath the Tyrrhenian, thin crust of oceanic type, the active subduction of the slab being witnessed by deep and very deep earthquakes; the north-central Apennines where the continental crust of the Adria microplate seems also subducted beneath the transitional, peri-Tyrrhenian type of crust but where the observed hypocenters are limited to the depth of about 100 km; the northern Apennines, where the same type of subduction seems to occur beneath the north-eastern slope of the mountain range, though evidenced by an even smaller number of events; finally, the western Alps: also here a small group of foci are recorded in the upper Mantle beneath the southern end of the “Ivrea body”. The different behavior of deep seismicity in the four areas confirms that the Italian peninsula is formed of sectors deriving from different geodynamical processes.